1. Field of the Invention
The present invention relates to an AC motor for driving a rolling mill and more particularly to a rolling mill driving AC motor which must be occasionally disassembled and reassembled and which is used under such a condition as permits only a limited axial space for drawing out a rotor.
2. DESCRIPTION OF THE PRIOR ART
Heretofore, as this type of a motor there has been proposed such a motor as illustrated in FIGS. 1A through 1C. More particularly, FIG. 1A is a front view of the aforementioned type of a rolling mill driving AC motor, in which, by way of simplification, only a top half portion of a stator and that of a rotor are shown, end bells are omitted and the details of a slip ring are also omitted. In the case of a squirrel-cage induction motor, the slip ring is not needed. FIGS. 1B and 1C are respectively partial left- and right-hand side views illustrating a positional relationship between a bed and a bearing pedestal in FIG. 1A. In FIG. 1A, the stator is composed of a stator active part 2 and a stator support 1 which supports the stator active part. On the other hand, the rotor is composed of a rotor active part, 3, a rotor spider 4 and a shaft 5. The rotor active part 3 is mounted on the rotor spider 4 and forms a so-called rotor. The rotor spider 4 is shrunk onto the shaft 5.
The rotor is supported through bearing pedestals 6 and 8 and the motor as a whole is mounted on a bed 10 which is a concreted integral bed. In the central portion of the bed 10 is formed a pit 10a having a length L.sub.1 in the axial direction of the shaft 5. The pit 10a is used for the connection of external leads or as a ventilation path.
The above conventional motor has the following drawbacks. First, for drawing out the rotor, it is necessary that the stator as an armature be divided horizontally in two, or when the rotor is to be drawn out in the axial direction, it is necessary that an extra space having a length almost equal to the overall length of the rotor be ensured in the axial direction. But, actually, these necessities cause a serious inconvenience, as will be described later in detail.
Conventional AC motors are generally operated at constant speeds and their installation space is not specially limited; besides, their rated speeds are relatively high, so even in the case of a large output motor, the size of the motor does not become so large. Consequently, even a large output motor could be easily installed and disassembled by the method in which first an integral stator is installed by means of a crane and then a rotor is inserted therein in the axial direction.
On the other hand, as rolling mill driving motors, DC motors have heretofore been used in most cases, which motors are generally low in speed (e.g. 6.5 MW 50/100 r.p.m.) and have a large overload capability in normal operation, e.g. 225% of rated base speed torque. Further, in the cases of slabbing mill and plate mill, a twin drive arrangement system is adopted, so the installation space is extremely limited. Moreover, in continuous hot strip mills five to seven finishing rolling stands (10-15 MW per stand) are arranged and therefore large-sized motors for driving those rolling mills are also disposed concentratively in a limited space.
In such rolling mill driving DC motors, it has been an essential requirement that the frame, namely, the stator, be divided in two in a horizontal position, for inspection and maintenance of the interior of the motor and in view of the crane capacity.
First, as to the inspection and maintenance of the interior of the motor, since rolling is performed under severe operational conditions involving quick acceleration, deceleration and reversal as well as mechanical and electrical impact load, there has been conducted a periodic inspection and maintenance of the interior of the stator and that of the rotor. Further, in the event of occurrence of a trouble in the interior of the motor, it is first required that the interior of the motor be opened quickly. The longer the duration of down time of the rolling operation, the more serious the expense, so in order to minimize the time required for inspection of the interior of the motor, it is absolutely necessary that the motor be opened quickly.
Next, in connection with the crane capacity, unless the frame or the stator is divided in two, horizontally, there is no space to draw out a rotor in the axial direction, and consequently the motor, including both the stator and the rotor, must be lifted integrally. In a hot reversing rolling mill such as slabbing mill or plate mill and where the motor is of a large torque, it requires more than 250 tons in weight of a motor. Consequently, it becomes necessary to use a crane of a large capacity, that is, an enormous amount of expense is required for building a housing structure strong enough to support such crane, for strengthening the piling foundation work comparable thereto, etc. Actually, therefore, the frame is divided in two horizontally, and the crane capacity is determined according to the heaviest piece (usually the rotor) among the rotor and the top and bottom frame portions.
Even without dividing the frame in two, a crane capacity according to only the rotor may suffice provided the rotor can be inserted in the axial direction. But, this is inapplicable to the case of a twin drive arrangement system in a hot reversing rolling mills. Also in the case of a continuous hot strip mill finishing stands, the following problems are involved. In the case of a single motor, the axial space doubles. In the case of a tandem motors, the axial length increases by a space corresponding to one motor, but in the case of taking out a front motor, all the rear motors must be removed before removal of the front motor. Therefore, in both the cases of a single and a tandem motors, the width of a motor room becomes larger; that is, an extra land is needed throughout the overall length of the motor room which is usually as large as 100 to 300 meters. Besides, this extra land is mostly a dead space, and the crane is required to have a larger span, which leads to an increased cost. Thus, there arise unacceptable problems.
In the case of using AC variable speed motors which have recently expanded their uses rapidly, such as induction motor and synchronous motor, in place of DC motors as main motors for driving rolling mills, either the foregoing requirement of dividing the frame in two as in the case of DC motors, or at least an equivalent function, must be satisfied. In this connection, the drawbacks of the prior art will be summarized below.
[1] In the case of dividing the frame:
Since DC motors are revolving-armature type, a mere division of the stationary field portion suffices in dividing the frame in two. But, AC motors are generally stationary-armature type, so the bisection of the frame is the very bisection of the armature portion. Usually, the armature voltage in a large-sized AC motors is high, and the armature coil in each armature slot comprises several-turn coils. Even assuming that the armature core can be easily divided in two, in order to divide the armature coil, at both front and rear coil end portions, on the right and left of the divided portion of the armature, it is necessary that the several-turn coils of every phase with a high voltage imposed thereon be bonded with bolts or by brazing. Consequently, not only a considerable time is required for disassembling and reassembling work, but also in point of reliability there may occur a bolt loosening at the live joints or burning of insulation materials near the brazing point, so this method is not considered to be a desirable method. Moreover, a long time (about 60 hours) is required for only disassembling and reassembling work, so it is absolutely impossible to finish a series of work such as disassembling, repairing and reassembling within the maximum allowable time (48 hours) as in DC motors for ordinary customers.
In the case of large water-wheel generator, the frame is divided into several portions for assembly at the installation site, but this is because of an ultra-large size thereof which makes the transport as a one body impossible. Once assembled at the installation site, it is not required that the frame will be disassembled for inspection, so such a large-sized waterwheel generator does not serve as a good reference.
[2]In the case of using a frame shifting device of a conventional standard type in synchronous motor:
The use of such a device is not acceptable for the foregoing reasons, for example, the limited space, but even if it could be mounted, it would result in increase of the axial length and tend to cause "lowering of torsional vibration frequency" which is a serious problem in the rolling use. Thus, the use of such a device is undesirable.